The present invention relates to a semiconductor device including a capacitor buried in an insulating film formed on a semiconductor substrate and a method of fabricating the semiconductor device.
In accordance with refinement of semiconductor devices such as a dynamic random access memory (DRAM), stored charge per unit area, namely, electrostatic capacity, has been increased by forming a capacitor three-dimensionally against a transistor by employing a stack type or trench type memory cell structure instead of a planar type structure.
Now, a conventional semiconductor device will be described with reference to FIG. 12.
As is shown in FIG. 12, on a semiconductor substrate 50 where a transistor (not shown) constituting a memory cell is formed, a first insulating film 51 is formed, and a plug 52 connected to the semiconductor substrate 50 (specifically, a diffused layer of the transistor) is formed in the first insulating film 51. The plug 52 includes a polysilicon film 52a and a barrier layer 52b successively buried in the first insulating film 51.
Furthermore, a second insulating film 53 is formed on the first insulating film 51, and a capacitor 54 connected to the plug 52 is formed in the second insulating film 53. The capacitor 54 includes a bottom electrode 54a, a capacitor dielectric film 54b and a top electrode 54c successively buried in the second insulating film 53.
In the conventional semiconductor device, since the barrier layer 52b is formed on the polysilicon film 52a in the plug 52, the polysilicon film 52a of the plug 52 and the bottom electrode 54a of the capacitor 54 can be prevented from being in contact with each other. As a result, the electric characteristic of the plug 52 can be prevented from degrading through oxidation of the polysilicon film 52a of the plug 52 during the formation of the capacitor 54.
In the conventional semiconductor device, however, there arises a first problem that the electric resistance of the plug 52 mainly including the polysilicon film 52a is increased as the diameter of the plug is reduced in accordance with refinement.
Moreover, the conventional semiconductor device has a second problem that the reliability of the capacitor 54 cannot be guaranteed because the bottom electrode 54 is contaminated or the first insulating film 51 or the second insulating film 53 is excessively etched during the formation of the capacitor 54.
In order to overcome the first problem, the present inventors have examined silicidation of a polysilicon film included in a plug for the purpose of reducing the resistance of the plug connected to the capacitor. Specifically, the polysilicon film included in the plug is silicided by using titanium.
Now, a method of siliciding a polysilicon film included in a plug by using titanium will be described with reference to FIGS. 13(a) through 13(d) and 14(a) through 14(d).
First, as is shown in FIG. 13(a), a contact hole 62 formed in a first interlayer insulating film 61 formed on a silicon substrate 60 is filled with a polysilicon film 63, and then, an upper portion of the polysilicon film 63 filled in the contact hole 62 is removed, thereby forming a recess 62a on the polysilicon film 63 in the contact hole 62 as is shown in FIG. 13(b).
Next, as is shown in FIG. 13(c), a titanium film 64 is deposited on the silicon substrate 60 so as to cover the top surface of the polysilicon film 63, and then, the titanium film 64 is subjected to a heat treatment for silicidation, thereby forming a titanium silicide layer 65 as is shown in FIG. 13(d).
The aspect ratio of the recess 62a is set to approximately 0.5 through 1.0 (namely, the recess has a depth of approximately 50 through 100 nm and a diameter of approximately 100 through 200 nm), so that a void cannot be formed within a barrier layer 67 subsequently formed on the titanium silicide layer 65 (as shown in FIG. 14(c). At this point, as is shown in FIG. 13(c), the titanium film 64 is continuously formed not only inside but also outside of the recess 62a. Furthermore, as is shown in FIG. 13(d), the titanium silicide layer 65 is formed not only in the surface portion of the polysilicon film 63 but also in the wall and the outside portion of the recess 62a . In other words, the titanium silicide layer 65 is formed also in the vicinity of the opening of the recess 62a, namely, in the vicinity of the opening of the contact hole 62.
Next, as is shown in FIG. 14(a), an unreacted portion of the titanium film 64 is selectively removed by wet etching, and, as is shown in FIG. 14(b), for example, a TiN film 66 is deposited on the titanium silicide layer 65 so as to completely bury the recess 62a.
Then, as is shown in FIG. 14(c), portions of the TiN film 66 and the titanium silicide layer 65 outside of the recess 62a are removed by the CMP, so that the barrier layer 67 can be formed from the TiN film 66 inside of the titanium silicide layer 65 within the recess 62a. In this manner, a plug 68 including the polysilicon film 63, the titanium silicide layer 65 and the barrier layer 67 is formed in the contact hole 62.
Next, as is shown in FIG. 14(d), a second interlayer insulating film 69 is deposited on the first interlayer insulating film 61, a recess 70 is formed in the second interlayer insulating film 69 so as to expose the top surface of the plug 68, and then, a conductive film 71 serving as a capacitor bottom electrode is deposited on the second interlayer insulating film 69 so as to cover the wall and the bottom of the recess 70.
Subsequently, although not shown in the drawings, a portion of the conductive film 71 outside of the recess 70 is removed, so as to form the capacitor bottom electrode from the conductive film 71 on the wall and the bottom of the recess 70, and thereafter, a capacitor dielectric film and a capacitor top electrode are successively formed on the capacitor bottom electrode.
In this fabrication method, however, the titanium silicide layer 65 and the conductive film 71 are directly in contact with each other (as shown in FIG. 14(d)) when the heat treatment is carried out for improving the electric characteristic of the conductive film 71 after the deposition thereof. Therefore, the titanium silicide layer 65 and the conductive film 71 are reacted with each other, resulting in siliciding the conductive film 71, namely, the capacitor bottom electrode. Furthermore, when the capacitor dielectric film including oxygen is formed on the capacitor bottom electrode, the titanium silicide layer 65 of the plug 68 is oxidized, which increases the resistance of the plug 68. As a result, the plug 68 cannot be utilized.